An inverter controlled DC arc welder is arranged to rectify AC power from a commercial power supply by an input rectifier, convert its DC output into high frequency AC power by an inverter composed of semiconductor switching elements, suitably reduce its voltage by an output transformer and again rectify it to obtain DC power for welding. The welder of this type has such advantages that the device can be made small in size and weight since it needs no input transformer of heavy weight, that the output transformer can be minimized by using an inverter providing a high frequency output, and that the output voltage of the welder can be stabilized effectively by controlling the switching operation of the inverter with a control voltage which can be derived from the difference between the output voltage of the welder and a predetermined reference voltage.
DC arc welding of consumable electrode type, such as CO.sub.2 welding, MAG welding and MIG welding, is effected by supplying such stabilized DC power as described above from a welder to a welding load formed of a continuously fed welding electrode rod and a base material to be welded. The welding electrode rod is brought into contact with or shorted to the base material and, then, separated from the base material to thereby form an arc therebetween. This shorting and separating operation is repeated. However, undesirable spattering may take place if a current supplied to the welding load becomes excessive when the electrode rod is shorted to the base material. In order to avoid such spattering, it has been a general practice to provide a large DC reactor in the output side of the welder which prevents rapid increase of the load current. However, the use of such a large DC reactor is a bar to reduction of the size and weight of the welder.
Means for preventing such undesirable spattering has been already proposed by people including some of the inventors of the present application in U.S. Pat. No. 5,306,894 and U.S. Pat. No. 5,272,314. According to U.S. Pat. No. 5,306,894, it is arranged that the periods of above-mentioned shorting and arcing are detected from a voltage between output terminals of the welder and the above-mentioned reference voltage is reduced by a predetermined amount during the shorting period to thereby prevent spattering. The reference voltage is raised by a predetermined amount during at least an early part of the arcing period to thereby prevent insufficient welding which would result from the above-mentioned reference voltage reduction.
According to U.S. Pat. No. 5,272,314, feedback control means is provided to detect the difference between the welder output voltage and a predetermined reference voltage. The inverter is controlled in accordance with the detected voltage difference to maintain the average output of the welder constant. At the same time, DC output current of the welder is detected, and the detected signal is differentiated. During a period in which the differentiated version of the detected signal is positive, or, in other words, during a shorting period, the reference voltage is lowered by a predetermined value so that the average output voltage becomes lower during the shorting period.
However, in the system of either one of the above-reference U.S. patents, the amount of correction to the reference voltage is always fixed and cannot follow unexpected change of welding load condition. More particularly, since the correction value is insensitive to unexpected changes in welding, such as movement of the welding electrode rod caused by improper handling by an operator and change in shape of portions to be welded, undesirable spattering may still happen. Especially, the system of U.S. Pat. No. 5,306,894 has such a problem that it cannot respond rapidly to changes in thickness of the base material, and, therefore, it is liable to cause blowout when thin plates are welded and insufficient weld when thick plates are welded.
Accordingly, an object of this invention is to provide an improved consumable electrode DC arc welder which can rapidly respond to any unexpected changes in load condition, for effectively preventing spattering and also avoiding blowout and insufficient welding.